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Following the congregation's silent recital of the Amida , the Sages instituted the ĥazan's out-loud repetition. This way, if anyone present does not know how to pray, they can fulfill the obligation by listening and responding “Amen” to each blessing. (There is no repetition in Ma’ariv since that service was originally optional.)
Even though prayerbooks are now readily available, and almost no one needs to fulfill the obligation by listening to the ĥazan, it is still customary for him to recite the repetition. In addition to the few individuals who may still need it, both recitations are necessary from a kabbalistic point of view– the silent recitation for the individual and the repetition for the community. Additionally, it is only during the repetition we recite Kedusha (within the third blessing). This responsive reading consists of three verses which speak of God’s holiness, and is traditionally recited standing with legs together.
When the ĥazan reaches the blessing of Modim, the congregation bows in unison with him, and recites a slightly different version of the prayer.
Before the last blessing of the repetition, any priests who are present walk to the front of the synagogue and recite the priestly blessing. In Israel, this takes place every day during Shaĥarit (as well as during Musaf on Shabbat and holidays). Outside Israel, the Ashkenazic custom is for the priests to recite the blessing only during Musaf on holidays.
